New cross sections for the reaction e p-->e p eta are reported for total center of mass energy W = 1.5--1.86 GeV and invariant momentum transfer Q2 = 0.25--1.5 (GeV/c)(2). This large kinematic range allows extraction of important new information about response functions, photocouplings, and eta N coupling strengths of baryon resonances. Newly observed structure at W approximately 1.65 GeV is shown to come from interference between S and P waves and can be interpreted with known resonances. Improved values are derived for the photon coupling amplitude for the S11(1535) resonance.

The most sensitive experiment to date to search for the muon and electron lepton number violating decay K 0 L ! m 6 e 7 has detected no events consistent with this process. Based on this result, the 90% confidence level upper limit on the branching fraction is B͑K 0 L ! m 6 e 7 ͒ , 4.7 3 10 212 . [S0031-9007(98)07985-X]

We have searched for a neutral H dibaryon decaying via H → Λn and H → Σ 0 n. Our search has yielded two candidate events from which we set an upper limit on the H production cross section. Normalizing to the inclusive Λ production cross section, we find (dσ H /dΩ) / (dσ Λ /dΩ) < 6.3 × 10 −6 at 90% C.L., for an H of mass ≈ 2.15 GeV/c 2 .

The masses of the K ~ and I ~ have been measured by use of exotic-atom x-ray transitions in Pb and W. The transitions used were free from contaminant nuclear y rays and strong-interaction effects. Two K~ transitions and three X~ transitions for each target satisfied these criteria. The results are Af^-=493.636 ±0.011 MeV and M L -= 1197.532±0.057 MeV. The precision of these measurements is greater than that of any previous experiment. When combined with previous measurements they form a consistent set.PACS numbers: 14.40. Aq, 14.20.Jn, 36.10.Gv Kaonic and Z-hyperonic exotic atoms in lead and tungsten have been studied with use of the Brookhaven National Laboratory Alternating Gradient Synchrotron. The magnetic moment of the Z~ obtained from the analysis of the earliest portion of these data has been published, and a final manuscript is in preparation. l In this Letter we present the results of the analysis of all the data for the K ~ and Z ~ masses.Kaons of 680 MeV/c momentum from the separated C4 beam (K~:K~ =9:1) were slowed down in a Cu degrader and brought to rest in a laminar target consisting of sheets of natural Pb or W immersed in liquid hydrogen. The Z ~ hyperons were produced by kaons stopped in liquid hydrogen through the reaction K~+p -> I,~7c*, which has a 46% branching ratio. The resulting monoenergetic n + (7^+=83 MeV) served as a tag for Z~ production Cr z = 12.4 MeV). The /r + were stopped in scintillation range spectrometers in which the subsequent e + from the decay chain K + -*/* + -* e* were detected. The Z ~ were brought to rest in the target material where they formed exotic atoms. Kaonic atoms were formed by beam K~ which stopped in the target sheets. Additional experimental details are given inRef. 1. X rays from the Z ~ and K ~ atoms were detected by three intrinsic reverse-electrode Ge detectors housed in a single cryostat beneath the target. The x-ray events were sorted into tagged and untagged histograms depending on whether a K* was detected in coincidence with a stopped K~ and a signal from one of the Ge detectors. Figure 1 shows a Pb untagged spectrum with intense kaonic x-ray lines while in the tagged spectrum, Fig. 2, the Z ~ x-ray intensities are enhanced.The Ge-detector resolution, line shape, and absolute energy calibration were determined continuously under beam conditions by our monitoring radioactive sources. The calibration sources 2 used were 57 Co, 133 Ba, 192 Ir, and 137 Cs. During beam-on periods chance coincidences between incident beam n ~ and y rays from the radioactive sources were recorded as beam-on calibration events. During the beam-off portion of the Alternating Gradient Synchrotron beam spill, histograms from the 122.061-and 661.660-keV lines from 57 Co and 137 Cs were accumulated. Every ten minutes these histograms were fitted on-line to provide a check on the gain stability of the systems and the results were written to tape for use in the off-line analysis as a two-point stabilization correction.The relative intensities of the unresolved x rays result...